docs: def add_transform_chain(self, tc):
          """Insert the GLSL snippets of a transform chain."""
          # Generate the transforms snippet.
          for t in tc.gpu_transforms:
              if isinstance(t, Clip):
                  # Set the varying value in the vertex shader.
                  self.insert_vert('v_temp_pos_tr = temp_pos_tr;')
                  continue
              self.insert_vert(t.glsl('temp_pos_tr'))
          # Clipping.
          clip = tc.get('Clip')
          if clip:
              self.insert_frag(clip.glsl('v_temp_pos_tr'), 'before_transforms')